The answer is that the traffic is routed through a NAT gateway, and flow logs are not enabled on the NAT gateway’s subnet. VPC Flow Logs capture traffic at the elastic network interface level, so logs on the EC2 instance’s subnet will only show traffic that passes through its own ENI; when traffic is sent to an external IP, it is forwarded to the NAT gateway, which performs source NAT and sends the packets out through its own ENI in a different subnet. This means the engineer must also enable flow logs on the subnet hosting the NAT gateway to see the outbound traffic to the external IP. On the AWS Certified Security Specialty SCS-C02 exam, this concept tests your understanding of how flow logs interact with managed network components—a common trap is assuming that enabling flow logs on the source subnet captures all egress traffic, but the NAT gateway acts as a separate hop. Memory tip: “Follow the ENI”—flow logs capture traffic only where the elastic network interface lives, so if the NAT gateway has its own ENI, you need logs on that subnet too.
SCS-C02 Security Logging and Monitoring Practice Question
This SCS-C02 practice question tests your understanding of security logging and monitoring. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A security engineer is investigating a potential data exfiltration incident. They suspect that an EC2 instance was compromised and used to transfer large amounts of data from an S3 bucket to an external IP address. The engineer has enabled VPC Flow Logs for the subnet where the EC2 instance resides, but the logs are not capturing traffic to the external IP. What is the most likely reason?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The traffic is routed through a NAT gateway, and flow logs are not enabled on the NAT gateway's subnet.
Option C is correct because VPC Flow Logs do not capture traffic to external IPs that is routed through a NATgateway if the flow log is configured only for the subnet where the EC2 instance is located; the NAT gateway's traffic is captured by flow logs on the subnet where the NAT gateway is. Option A is wrong because Flow Logs can capture accepted traffic. Option B is wrong because Flow Logs capture traffic even if Security Groups are present. Option D is wrong because Flow Logs capture all IP traffic, not just intra-VPC.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The EC2 instance's security group is blocking the traffic.
Why it's wrong here
Security groups do not affect Flow Logs; they log traffic regardless.
✗
VPC Flow Logs only capture rejected traffic.
Why it's wrong here
Flow Logs can capture both accepted and rejected traffic.
✗
VPC Flow Logs only capture traffic to and from other VPC resources.
Why it's wrong here
Flow Logs capture all IP traffic, including to external IPs.
✓
The traffic is routed through a NAT gateway, and flow logs are not enabled on the NAT gateway's subnet.
Why this is correct
Flow Logs need to be enabled on the subnet where the NATgateway resides to capture its traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Security Logging and Monitoring — This question tests Security Logging and Monitoring — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The traffic is routed through a NAT gateway, and flow logs are not enabled on the NAT gateway's subnet. — Option C is correct because VPC Flow Logs do not capture traffic to external IPs that is routed through a NAT gateway if the flow log is configured only for the subnet where the EC2 instance is located; the NAT gateway's traffic is captured by flow logs on the subnet where the NAT gateway is. Option A is wrong because Flow Logs can capture accepted traffic. Option B is wrong because Flow Logs capture traffic even if Security Groups are present. Option D is wrong because Flow Logs capture all IP traffic, not just intra-VPC.
What should I do if I get this SCS-C02 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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